Apr 8, 2010 12:00 PM, By Bob McCarthy
There are other options beyond the broadband drying of the room that results from panel absorbers. Fiberglass and soft-foam absorber systems are much more effective as frequency rises, leaving the room with much shorter decays in the HF range. Tuned devices can be targeted for particular ranges, most notably the low frequencies, enabling for a more balanced spectrum in the reverberant decay characteristic.
The original name (since Helmholtz never developed a commercial line) in commercially available tuned absorbers is the Tube Trap . These devices, manufactured by Acoustic Sciences (ASC), look like furniture from a Stanley Kubrick film. Cylinders of various lengths and radii can be placed near walls and corners to neutralize a target frequency range. The cylinders contain a mixture of diffractive and absorptive elements, thereby creating a tuned antiresonator for the low-frequency range and diffraction and absorption above that. In addition to the full cylinders, the company now offers half-rounds and quarter-rounds, allowing for more options in placement, particularly for corners.
Another player in trapping technology is RealTraps. The company combines the direct sound absorptive properties of a traditional panel absorber with the flatpanel resonating-membrane absorber into a single device. The combination extends the range of action down in frequency to create an even spectrum of absorption. The newest entry in the line is the MegaTrap, a corner-placed limp-mass membrane unit that is nearly 3ft. wide. It is claimed to absorb more and to lower frequencies than any other absorber. MegaTraps have the aesthetic advantage of appearing to be simple fabric panel and are available with the front fabric in white, wheat, gray, and black colors.
Foam absorbers placed in the corner can also create a bass trap. Auralex Acoustics has a mega product of its own: the MegaLENRD bass trap. For the acronym-impaired, LENRD stands for “low-end node-reduction device,” “node” being a term related to fixed-location resonance activity of low frequencies in a room. Corners are the most critical location for nodal control. The MegaLENRD is the largest of this family of easy-to-install foam corner absorbers and is suitable for studio as well as larger-scale applications where visible foam is acceptable.
The third major classification of room acoustic treatment products is the diffuser. These devices aim to scatter the sound from their complex faceted surfaces to create a more randomized reverberation character. One can visualize the complexities of the ornate plaster walls in traditional concert halls and realize the acoustic value of the art was in its quality of diffusion. For those of us without budget or desire for plaster artwork, there is a great variety of modern products mathematically engineered to provide controlled scatter. A well-designed space can blend absorption and diffusion products effectively to create a room with sufficient reverberation, free from flutter echoes (detectible and consistently time-separated reflections). Next, we will look at some exclusively diffusive products and some that are hybrids of absorption and diffusion.
RPG has developed the Binary Amplitude Diffsorbor (BAD): a diffusing absorber that is flat in appearance and no deeper than a purely absorptive panel. A BAD panel simultaneously provides uniform diffusion at high- and midband frequencies and then pure absorption below the diffusive range. The energy that is not diffused is absorbed.
The finished appearance of the panel matches a traditional fabric-covered absorption panel, yet these units contain an intermediate layer between the fabric and fiberglass. The diffraction is carried out by a uniquely patterned perforated rigid panel that scatters the highs and allows the lows to pass through to the absorber behind. The binary hole pattern resembles a Lite-Brite in appearance, but it is mathematically calculated to provide a flat power spectrum of diffuse energy. These panels can flush mount to walls or ceilings with a wide assortment of fabrics available.
Acoustics First has developed the Art Diffusor, a patented series of two-dimensional binary array diffusers. The diffusion range of the Art Diffusor is extended by its unique angled end caps to further control detectible specular reflections above 4kHz. The various models differ in their depth and material. As the depth increases from 2in. to 9in., the frequency span where diffusive qualities dominate ranges from 2 octaves to 5 octaves. They are available in furniture-grade wood or expanded polystyrene.
The AlphaSorb sound diffusers by Acoustical Solutions discreetly provide diffusion while having the appearance of a fabric-covered wall or ceiling panel. Behind the fabric are specially molded plastic diffusers designed to scatter or disperse acoustic energy. They are 2in. thick and available in 2’x2’, 2’x4’, and 4’x4’ sizes and a variety of colors. Users can also create the mixed acoustical effects of diffusion and absorption by combining the AlphaSorb sound diffusers with a pure absorber, the visually matched 2in.-thick AlphaSorb acoustical wall panels.
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